1. Field of the Invention
The present invention relates to a pre-combustion chamber-type engine with a pre-combustion chamber structure installed in a cavity formed in a cylinder head or piston.
2. Background of the Invention
A conventional pre-combustion chamber-type engine has a construction, in which a cylinder head provided with intake and exhaust ports is formed with a cavity; in which a hot plug having a communication hole to communicate the pre-combustion chamber and the main combustion chamber is fitted in the lower part of the cavity; and in which fuel is injected into the pre-combustion chamber from a fuel injection nozzle. In another pre-combustion chamber-type engine, a pre-combustion chamber structure, which forms a pre-combustion chamber and has a communication hole communicating the pre-combustion chamber and the main combustion chamber, is installed in a cavity formed in the cylinder head, with fuel injected into the pre-combustion chamber from the fuel injection nozzle. Such pre-combustion chamber-type engines are disclosed in, for example, Japan Utility Model Laid-Open Nos. 117149/1992 and 79019/1983.
The pre-combustion chamber type-engine disclosed in Japan Utility Model Laid-Open No. 117149/1992 has formed thin the fuel impinging portion of the pre-combustion chamber wall body in the cylinder head to reduce the heat capacity of the fuel impinging portion to enable rapid temperature increase. In this pre-combustion chamber type-engine, the portion of the pre-combustion chamber wall body that the fuel jet from the fuel injection nozzle strikes is formed thinner than other portions, the outer side of the pre-combustion chamber wall body is recessed to reduce the heat capacity, and a heat insulating layer is formed between the recessed portion and the wall surface of the hole portion of the cylinder head.
In the pre-combustion chamber type engine disclosed in Japan Utility Model Laid-Open No. 79019/1983, the pre-combustion chamber member, fitted in a cavity or receiving hole formed in the cylinder head, is divided into an upper pre-combustion chamber member made of a ceramics material and forming the upper part of the pre-combustion chamber and a lower pre-combustion chamber member made of a ceramics material and forming the lower part of the pre-combustion chamber. A gasket, which has an elasticity in the direction of insertion to the receiving hole, is installed in a joint portion between the upper and lower pre-combustion chamber members.
In recent years, a pre-combustion chamber type engine has become available, in which a cavity is formed in the piston installed slidably movable in the cylinder, and in which a pre-combustion chamber structure, that forms the pre-combustion chamber and has a communication hole to communicate the pre-combustion chamber and the main combustion chamber and a hole into which a fuel injection nozzle is inserted, is arranged in the cavity, with fuel injected into the pre-combustion chamber.
In diesel engines, it is known that the engines' theoretical heat efficiency, i.e., the designated average effective pressure P.sub.mi, changes greatly according to the compression ratio as shown in FIG. 11. The higher the compression ratio, the better the designated average effective pressure. As the compression ratio is increased, however, the pressure in the cylinder increases, causing the mechanical loss, i.e., the friction average effective pressure P.sub.mf, to rise. The net average effective pressure P.sub.me which represents the fuel efficiency of the engine (virtual heat efficiency) does not increase proportionately with the compression ratio, but levels off at a certain point. Hence, in the diesel engine, the best thermal efficiency is obtained when the compression ratio is in the shaded range of 18.5 to 20.5 in FIG. 11.
In the conventional 1600-cc 4-cylinder pre-combustion chamber type diesel engines, the compression ratio is normally around 22 (see FIG. 12). In the 1600-cc-4-cylinder pre-combustion chamber type diesel engines, the highest thermal efficiency is theoretically obtained when the compression ratio is close to 19. The optimum compression ratio as related to the displacement per cylinder changes as indicated by the curve E. With the diesel engines, however, setting the compression ratio to 19 makes the cold start difficult and it is unavoidable to set the compression ratio high to secure good starting performance, which in turn lowers the thermal efficiency.
FIG. 12 shows the relation between the displacement per cylinder and the compression ratio in the pre-combustion chamber type engine. FIG. 12 plots such relations with circular, triangular and square marks, for a Case of the conventional pre-combustion chamber type engine that was started within a cranking time of 10 seconds. It is seen that the conventional pre-combustion chamber-type engine has a feature almost following the curve G. The curve G is the result of having to set the compression ratio unnecessarily high to secure the cold start. The conventional pre-combustion chamber type engine is therefore considered to have the necessary starting performance. The curve G, as described later, can be given by the following general expression. EQU y=0.03579.sup.z -2.2
where Z=1/(-0/01029x.sup.1/3 -1).
The pre-combustion chamber type diesel engine (IDI) requires a high compression ratio compared with the direct injection type diesel engine (DI), as shown in FIG. 13. This is to secure the compression end temperature required for ignition during cold starting of the diesel engine. That is, the pre-combustion chamber type diesel engine has a large inner wall surface area of the combustion chamber and therefore a large heat dissipating area compared with the direct injection type diesel engine. Furthermore, because the air flow in the pre-combustion chamber, particularly in the swirl chamber, is very active, the heat conduction of the pre-combustion chamber inner wall portion is high and the flux of heat being dissipated is large. In the pre-combustion chamber type diesel engine, therefore, it is not possible to select the compression ratio that makes the net average effective pressure P.sub.me maximum (in FIG. 13, indicated by MaxP.sub.me), and the use of high compression ratio lowers the fuel efficiency.
In general pre-combustion chamber-type engines, only the lower part of the pre-combustion chamber is formed of a heat insulating material and the upper part is provided by the body of the cylinder head. The upper part of the pre-combustion chamber, which receives heat from the hot compressed air during starting, is in direct contact with cooling water and thus its temperature hardly increases. The lower part of the pre-combustion chamber is in many cases fitted under pressure into the cylinder head, and heat flows from the pre-combustion chamber to the cylinder head body to the cooling water, with the result that the temperature rise is very slow. Further, in the pre-combustion chamber type engine whose entire pre-combustion chamber is formed of a heat insulating material, the pre-combustion chamber structure is generally fitted under pressure into the cylinder head body and heat flows from the pre-combustion chamber to the cylinder head to the cooling water, so that the temperature increase is slow. Because the member forming the entire pre-combustion chamber has a large heat capacity, the temperature rise of the pre-combustion chamber is slow even if the heat flow from the pre-combustion chamber to the cylinder head is cut off. And the bad starting performance makes it necessary to increase the compression ratio.
Generally, the diesel engine uses a glow plug as an assist to secure reliable starting. Because the cranking time is 10 seconds at most, when more than 10 seconds of cranking is required, it is decided that the engine has failed to start.